You only need it if you want to know how your disk susbsystem is performing, or you want to troubleshoot disk related performance problems, or you want to establish a baseline performance metric for the disk susbsystem. If none of those things are your objective, then you don't need it.

Your question is more likely to get quality answers phrased like this "Why would we want to graph HDD reads\writes?" as opposed to "Why do we need to graph HDD reads\writes?" because like I said, you don't need it... but you may want it...

Like all the rest of the graphs you see, it's primarily relevant in the context of history. If the averages stay around the same for several weeks or months and then quickly spike or drop at the same time as you notice a distinct change in performance, the graph may help you spot what's changed.

If performance goes down and disk IO goes up, then your performance decrease is probably caused by amount of information that must be read/written to perform a specific task. But if performance goes down and disk IO goes down, then your problem probably has to do with something that's preventing or slowing that IO from happening -- perhaps deadlock or disk errors.